Television systems have been proposed which use sub-Nyquist sampling of the luminance signal for obtaining increased resolution through frequency interleaving, while keeping the frequency spectrum occupied by the luminance information the same, and for achieving at the same time some degree of backward compatibility with existing television receivers. In sub-Nyquist sampling of a progressively scanned television signal, in a first set of alternate frames every odd sample in odd-numbered lines is replaced by a zero and every even sample in even-numbered lines is replaced by a zero, in a second set of alternate frames (interleaved in time with the first set) every even sample in odd-numbered lines is replaced by a zero and every odd sample in even-numbered lines is replaced by a zero, and the zero samples are not transmitted. Field-interlaced television signals can also be sub-Nyquist sampled. The sub-Nyquist sampling of a luminance signal of up to twice the bandwidth of a conventional broadcast luminance signal generates a "folded-luminance" signal with a bandwidth no more than that of a conventional broadcast luminance signal.
E. A. Howson and D. A. Bell describe frequency interleaving of luminance information in the analog domain in an article "Reduction of Television Bandwidth By Frequency Interlace" in pages 127-136 of the February 1960 Journal of the British Institute of Radio Engineers. In one scheme envisioned by Howson et alii the entire luminance signal is used to amplitude-modulate a carrier having a frequency just above the highest frequencies in the luminance signal, and the resulting signal is supplied to a low-pass filter having a cut-off frequency at mid-band. In another scheme envisioned by Howson et alii the luminance signal is separated by a band-splitting filter into two components residing in respective ones of equal-bandwidth low-frequency and high-frequency bands, the high-frequency-band component is used to amplitude-modulate a carrier having a frequency just above the high-frequency band, and the lower sideband resulting from the amplitude modulation is combined with the low-frequency-band component to obtain a folded-luminance signal. Reducing television bandwidth by frequency interlace cannot be satisfactorily implemented in the analog domain, however, because of the difficulty of removing artifacts introduced by the frequency interleaving that manifest themselves as annoying dot crawl in the reconstructed full-band television image.
There have been proposals to use sub-Nyquist sampling of the luminance signal to improve home video cassette recorders (VCRs) using the VHS format. The VHS format is a color-under format. The chrominance information is recorded as in-phase and quadrature amplitude-modulated sidebands of a suppressed 629 kHz carrier, which is the fortieth harmonic of horizontal synchronization rate. The luminance information is recorded as frequency modulation of a carrier which can vary in frequency from 3.4 to 4.4 MHz (m 0.1 MHz), to occupy a 1.4-7.0 MHz band after filtering to suppress sideband energy below 1.4 MHz. The luminance and chrominance carriers are recorded and played back using helically scanning heads mounted in a rotating headwheel assembly, with these two components of video information being recorded on diagonal tracks. Stationary heads can be used for recording and playing back sound information recorded in lateral sound tracks on the video tapes. Alternatively, high-fidelity stereophonic sound can be recorded on and played back from deep diagonally recorded sound tracks by helical scanning procedures implemented by wide-gap heads also included in the rotating headwheel assembly. The high-fidelity stereophonic sound frequency-modulates a 1.2 MHz carrier.
One such modification of VHS format video tape recording was proposed by Faroudja in U.S. Pat. No. 4,831,463 issued 16 May 1989 and entitled "VIDEO PROCESSING IN WHICH HIGH FREQUENCY LUMINANCE COMPONENTS ARE FOLDED INTO A MID-BAND SPECTRUM". In the Faroudja video recording system (according to column 9, lines 30-35, of U.S. Pat. No. 4,831,463) the sub-Nyquist folding frequency is carefully chosen from amongst those frequencies which are precise harmonics of an odd multiple of both the line and frame scan rates of the baseband luminance. Faroudja performs sub-Nyquist sampling on the entire luminance signal, sampling at a folding clock frequency rate to generate a reversed frequency spectrum frequency-translated to baseband, there to interleave with the original frequency spectrum, and then applies the result to a low-pass filter cutting off at one-half the folding clock frequency to generate the folded-luminance signal. This procedure amplitude-modulates the folding clock frequency in a modulation procedure that--though it may be balanced with regard to suppressing the folding clock frequency--cannot be balanced with regard to modulating signal.
Another modification of VHS format video tape recording was briefly described by C. H. Strolle, J. W. Ko and Y. J. Kim in their paper "A Compatibly Improved VHS System" appearing on pages 122-123 of IEEE 1991 International Conference on Consumer Electronics Digest of Technical Papers for a conference held 5-7 Jun. 1991 in Rosemont, Ill. This improved VHS video recording system is described in more detail in the U.S. patent application Ser. No. 787,690 filed by Christopher H. Strolle et alii, entitled "SYSTEM FOR RECORDING AND REPRODUCING A WIDE BANDWIDTH VIDEO SIGNAL VIA A NARROW BANDWIDTH MEDIUM" and assigned to Samsung Electronics. In this improved video recording system the sub-Nyquist folding carrier is chosen to be 320 times line scan rate, rather than one of the frequencies specified by Faroudja in U.S. Pat. No. 4,831,463.
To avoid generating aliasing artifacts that are intolerable to a person viewing a television image recovered from the video signal sampled at sub-Nyquist rate, before recording the luminance signal, the video recording system described in U.S. patent application Ser. No. 787,690 processes luminance as described immediately hereinafter. A band-splitting filter is used to separate a spatio-temporally filtered luminance signal into low-frequency-band and high-frequency-band spectra. The high-frequency-band spectrum is adaptively de-emphasized, or reduced in amplitude respective to the low-frequency-band spectrum.
If after its de-emphasis the high-frequency-band spectrum were re-combined with the low-frequency-band spectrum to generate a full-band luminance signal with de-emphasized high frequency content, the generation of folded-luminance signal from that full-band signal by the method Faroudja uses introduces a problem during playback with regard to restoring the de-emphasized high-frequency-band spectrum to its original amplitude respective to the low-frequency-band spectrum. This problem arises because, in addition to the roll-off of the lower frequencies of the high-frequency-band spectrum caused by the band-splitting filter used to separate the spatio-temporally filtered luminance signal into low-frequency-band and high-frequency-band spectra, there is an additional roll-off of the lower frequencies of the high-frequency-band spectrum caused by the low-pass filter used in generating the folded-luminance signal from the sub-Nyquist-sampled full-band signal.
This additional roll-off is avoided in the video recording system described in U.S. patent application Ser. No. 787,690 by applying the de-emphasized high-frequency-band spectrum to a balanced modulator, wherein that modulating signal is heterodyned with the folding carrier. The modulation procedure is one that suppresses modulating signal as well as folding clock frequency in the modulation result. The resulting balanced modulator output signal is a reversed spectrum frequency-translated to baseband and unaccompanied by the original spectrum of the modulating signal. This reversed spectrum, which encodes the de-emphasized high-frequency-band spectrum, has only the roll-off of the original band-splitting filter. There is an absence of significant spectral energy in the frequency range below the folding clock frequency occupied by the original de-emphasized high-frequency-band spectrum or in the mirrored frequency range above the folding clock frequency. Above the folding clock frequency there is also an absence of significant spectral energy in the mirror of that frequency range. That is, the balanced modulator output signal is free of first-harmonic (and all other odd-harmonic) sidebands of the folding clock frequency. The balanced modulator output signal is added to the low-frequency-band spectrum to generate a folded-luminance signal. It is then easy for one skilled in the art of filter design to design a low-pass filter for suppressing the even-harmonic sidebands of the folding clock frequency generated by the balanced modulator, while not introducing objectionable additional roll-off of the folded-luminance signal near the cross-over frequency of the band-splitting filter.
The folded-luminance signal is supplied as a modulating signal to a frequency modulator. A frequency-modulated luminance carrier generated by the frequency modulator is combined with a complex-amplitude-modulation color-under carrier to generate a recording signal for the helically scanning video record head(s).
During playback the video recording system described in U.S. patent application Ser. No. 787,690 operates in the following manner. The frequency-modulated luminance carrier and the complex-amplitude-modulation color-under carrier in the playback signal recovered by the helically scanning video playback head(s) are separated from each other by respective band filters before each is demodulated. The folded-luminance signal is recovered by detecting the frequency modulation of the luminance carrier and then subjecting the detected folded-luminance signal to an unfolding procedure. In this procedure the folded-luminance signal is supplied as a modulating signal to a modulator, to be heterodyned with the folding carrier to restore its reversed spectrum portion to its original high-frequency band. This heterodyning procedure generates an undesirable image of the low-frequency band accompanying the high-frequency band in its portion of the spectrum. The low-frequency band, the restored high-frequency band and the high-frequency-band image of the low-frequency band are spatio-temporally filtered to suppress the image, thus to recover the luminance signal with a de-emphasized high-frequency-band.
The de-emphasized high-frequency-band spectrum is separated from the low-frequency band by a band-splitting filter and re-emphasized to restore its original amplitude vis-a-vis the amplitude of the original baseband spectrum. It is this restoration step that would be hampered by roll-off of the lower frequencies of the high-frequency-band spectrum that is additional to that associated with the band-splitting filter used during recording. Such roll-off would tend to introduce a reduction in the amplitude of of mid-spectrum frequencies.
The amplitude-restored high-frequency-band spectrum is then rejoined with the low-frequency band from which it was separated, thereby to recover a full-bandwidth replica of the wide-bandwidth luminance signal. This wide-bandwidth luminance signal, the chrominance signals demodulated from the complex-amplitude-modulation color-under carrier, and the sound signal recovered from the audio tape track are suitable for being encoded in substantial accordance with a broadcast color television standard and used to amplitude-modulate a broadcast-band carrier wave, thereby to generate signals suitable for application as input signals to a color television broadcast receiver. Alternatively, this wide-bandwidth luminance signal, the chrominance signals, and the sound signal may be supplied directly to a color television monitor, rather than being used to amplitude-modulate a broadcast-band carrier wave.
The generation of the folded video signal proceeding from a wide-bandwidth luminance signal and the regeneration of the wide-bandwidth luminance signal proceeding from the folded video signal are procedures best carried out in the digital, rather than analog, regime. Generally, a balanced modulator is realized in the digital regime as follows. A pair of four-quadrant digital multipliers are used for multiplying samples of a digitized modulating signal by samples of a first digitized carrier wave and by samples of a second digitized carrier wave opposite in phase to the first, and a digital adder combines the two digital products that result to generate the output signal of the balanced modulator. Digital multipliers that operate at video sample rates are expensive, taking up large areas on a monolithic integrated circuit. Such multipliers consume substantial power and so present problems with getting rid of the heat they generate.
Choosing the luminance sampling rate to be twice the folding carrier frequency in a folded-luminance signal employed in a video recording system causes the lower of the first-harmonic sidebands of the sampling frequency to fall into a frequency range immediately above the baseband spectrum. This would appear to interfere with the folding of the luminance spectrum. The high-luminance-frequency spectra in the baseband and in the lower first-harmonic sideband combine, however, to provide a frequency spectrum that is the equivalent of a band-pass filter response centered at the folding carrier frequency. A balanced modulator operated at folding carrier frequency responds to the band-pass filter response centered at the folding carrier frequency to generate an output signal in which the two high-luminance-frequency spectra fold together around zero frequency to form a reversed high-luminance-frequency spectrum, without the expected interference arising. Choosing the luminance sampling rate to be twice the folding carrier frequency in a folded-luminance signal employed in a video recording system reduces balanced modulation of a suitably phased folding carrier wave to multiplication of successive samples of the modulating signal alternately by plus one and by minus one. Such multiplications are done without having to use a digital multiplier.